This invention relates to an optical element having an optical surface with a specified surface roughness, an optical system including this optical element, an optical pickup device, and a molding die capable of obtaining the optical element.
In recent years, in the high-density recording based on an high-density optical recording medium of gigabyte order such as various kinds of DVD's and a large-capacity MO which have been being actively brought into practice, the pickup optical system is made to have a remarkably high precision, a high function, and a high degree of difficulty. In particular, in the aspherical objective lens which is one of the key parts of the device, heretofore, it has been generally required to make its NA high for improving light collecting ability, to make high the precision of the shape of its optical surface because of the wavelength of the light from the light source being made short, and efforts for improving them have been made.
On the other hand, as a result of diligent studies, the inventors have recognized that a sufficient optical performance can not be obtained by only the above-described high NA and high precision of the shape of the optical surface, and especially, in the case where the wavelength of the light source becomes 500 nm or less, they have discovered that the influence of the surface roughness of the optical element becomes very large, and also it becomes large the influence to the performance of reading and recording of the optical information recording medium. Thus, on the basis of their finding, the inventors come to think of this invention. The detail will be described below.
In particular, with respect to Rayleigh scattering, which is produced by fine particles having a size of the same order as the light wavelength, the amount of scattered light is basically large for a coherent light like a laser beam, and it becomes large in inverse proportion to the fourth power of the wavelength of the used light source; therefore, a remarkable loss of light quantity comes to be produced by fine ruggedness and dusts on the optical surface when the wavelength is short. This makes a cause of serious bad operations such as the decrease of contrast of the focus spot owing to the scattered stray light and the occurrence of an error owing to the deficient light quantity at the time of reading and writing of an optical disk. As the countermeasure of it, it is thinkable to use a semiconductor laser having higher output for the light source; however, because a semiconductor laser becomes very high-priced and has a low reliability for a large output, it is not a suitable method to use it in an optical system for a pickup for which a high reliability is required. With regard to Rayleigh scattering, the intensities of the scattered light at 650 nm, which is the wavelength of a light source used for a DVD, 500 nm, and 400 nm, which are the wavelengths of light sources for optical disks of the next generation are shown in the following Table 1, in which the intensity of the scattered light at 780 nm, which is the wavelength of a light source used for a conventional CD, is given a value 1.0 for the same surface roughness as those.
TABLE 1Wavelength of used light source (nm)780650500400Rayleigh scattering(1)2.15.914.5
From Table 1, it can be understood that Rayleigh scattering increases more suddenly when the wavelength of the used light source comes to the neighborhood of 500 nm or less.
As described in the above, in the case where the wavelength of the light source becomes 500 nm or less, because the influence of the surface roughness of the optical element becomes very large, and also it becomes large the influence given to the performance of reading and recording of the optical information recording medium, the inventors come to make this invention by specify a desirable surface roughness of the optical element. Further, in the case where information is recorded on an optical information recording medium, it is desirable, especially in the optical system, to reduce the loss of light quantity.